1. Technical Field
The present invention relates to a lighting control circuit for vehicle lighting equipment and, more particularly, a lighting control circuit for vehicle lighting equipment constructed to control the lightening of a semiconductor light source that is formed of a semiconductor light emitting device.
2. Related Art
In the prior art, as the vehicle lighting equipment, the equipment using a semiconductor light emitting device such as LED (Light Emitting Diode), or the like as a light source is known. Also, the vehicle lighting equipment of this type is equipped with a lightening control circuit that controls the lighting of the LED.
In northern Europe, North America, and others, particularly in the district where an amount of solar radiation is small even in the daytime of winter, the driver is bound to turn on the headlamp of his or her car in the daytime, i.e., DRL (Daytime Running Light). Therefore, in the vehicle such as the car, or the like sold in these countries, the lamp control system that is used to turn on the headlamp in a dimmed lighting mode even in the daytime is employed as (see JP-A-10-86746 (page 2 to page 5, FIG. 1)
In turning on the headlamp of the vehicle as the DRL, there are two ways, i.e., the way of using the dedicated lamp as such headlamp and the way of applying the existing lamp to such headlamp. When the bulb containing a filament that is made of halogen, or the like, for example, is used as the dedicated lamp, such bulb can be turned on in a dimmed lighting mode if the bulb is caused to emit a light at the designated brightness. When the LED is used as the dedicated lamp, such LED can be turned on in a dimmed lighting mode if a predetermined power or current is supplied to the LED.
On the contrary, when the existing lamp is also applied to such headlamp, for example, when the low-beam or high-beam headlamp is also applied to such headlamp, a quantity of light emitted is too much when these lamps are turned on in a full lighting mode. Therefore, from the viewpoint of energy saving, for example, these lamps can be turned on in a dimmed lighting mode using a PWM (Pulse Width Modulation) signal. The PWM signal is such a signal that has a frequency in a range of several hundreds Hz to several tens kHz, for example, and turns on/off a power (voltage/current) request at a certain duty ratio. When a duty ratio of the PWM signal is set to 50%, half of the power required in the full lighting mode is put into the lamp, and thus the brightness emitted from the lamp gives a quantity of light correspondingly. In case the lamp is turned on in a dimmed lighting mode using the PWM signal, either the bulb using the halogen filament or the LED can be employed as the lamp.
In controlling the lighting of the LED, the control signal composed of the PWM signal with a previously set duty ratio is supplied to the semiconductor switching device to turn on/off the semiconductor switching device at the set duty ratio when dimmed lighting conditions are satisfied, so that the LED is turned on to emit a quantity of light that corresponds to the set duty ratio. In this case, sometimes it is requested to employ the binary signal having High/Low values as the control signal when the LED is turned on in a full lighting mode or turned off. Therefore, upon constructing the lighting control circuit for the vehicle lighting equipment to control the lighting of the LED, the lighting of the LED must be controlled by discriminating the control signal. In addition, if the power supply circuit used to control the lighting of the LED or the switching regulator is directly connected to the battery power supply, a minute dark current flows through the power supply circuit or the switching regulator depending on the circuit configuration of the power supply circuit or the switching regulator the moment the LED is turned off. As a result, there is a possibility that the battery voltage is wasted.